1. Field of the Invention
This invention relates to an apparatus for compensating for focus error occurring in a device for projecting a negative onto a photosensitive member. More particularly, it relates to an apparatus for correcting focus error occurring in the projection system of a mask aligner when an integrated circuit pattern on a photomask or a reticle is transferred onto a wafer during the semiconductor manufacturing process.
2. Description of the Prior Art
In recent years, the advancement in the manufacture of patterns of semiconductor elements such as IC, LSI and VLSI toward minuteness and higher integration has been remarkable and the pattern line width required is approaching 1 to 2 .mu.m. What is required to achieve minute patterns with higher integration is the provision of an exposure device having a printing function of printing minute patterns of 1 to 2 .mu.m and an aligning function of accurately aligning each pattern in a plurality of steps of the manufacturing process as well as provide for a device that causes less defects to the wafer.
To meet these requirements, there have been developed various projection exposure devices (the reduction lens projection type, the one-to-one magnification lens projection type, the mirror projection type, etc.). The resolving power of a projection exposure device capable of printing minute patterns of 1 to 2 .mu.m is determined by the effective F-number of the projection optical system thereof and the wavelength .lambda. of the printing light. The depth of focus of such a device is defined by .DELTA.Z=.+-.2.lambda.Fe.sup.2.
In the case of reduction projection exposure devices having the printing function of 1 .mu.m which are presently provided, the printing light utilized has a wave light of 0.436 .mu.m. The device uses an optical system on the order of Fe=1.4 and the depth of focus matching this is on the order of .+-.1.7 .mu.m. Therefore, in these projection exposure devices, it is a requisite condition to have a focus adjusting mechanism for reliably imaging a photomask pattern on the surface of a wafer, and various methods therefor have heretofore been provided.
The focus adjusting mechanisms presently provided may be generally classified into two types. A first type is the T.T.L. type in which the position of a wafer is detected through a projection optical system and the wafer is always adjusted to the best imaging position. A second type is the constant distance type in which a wafer is always adjusted at a constant distance relative to a projection optical system. The T.T.L. type system requires a complicated optical system, suffers from various limitations in the design of the projection optical system and thus, at present, most projection exposure devices adopt the constant distance type system.
The wafer position detecting methods of the constant distance type presently provided include the method using an air micrometer, the method using an electrical non-contact micrometer, and the method using an optical system. Any of these methods has a wafer position detection accuracy on the order of .+-.0.3 .mu.m and is combined with a focus adjusting mechanism so as to be sufficiently able to set a wafer always at a constant distance capable of satisfying the depth of focus relative to the projection optical system.
However, the constant distance type system suffers from the following disadvantage. In the case of the constant distance type system, a wafer is always set at a constant distance relative to the projection optical system and therefore, to ensure that the wafer lies at the best imaging position, it is a necessary that the best imaging position of the projection optical system do not vary.
However, it is generally known that when a printing light is applied to the projection optical system to effect the printing of the wafer, the projection optical system absorbs part of the printing light and the optical performance thereof varies with heat and the best imaging position is displaced. It is for this reason that development of an optical system which will suffer less from variation in optical performance due to the application of the printing light is desired.